Implements monitoring-network

This feature collects the network information(devices, connect statuses
and statistices) via North Bound API from the SDN controller.

Implements: blueprint monitoring-network

Change-Id: Ifaa88fd3135dafe8ae3e83e03b76fb9b7e1b0904
rename meter table to sample

as we branch common meter definition data to it's own table,
we rename table containing raw data to more appropriate name:
sample

blueprint: meter-table-sql
Change-Id: I55c86540098225a3d52116df8b27854483921ee9
sample table contains redundant/duplicate data

create a meter table to store meter definition attributes:
- name
- type
- unit

blueprint: meter-table-sql
Closes-Bug: #1211985
Change-Id: I4dd510e66d64226b03cfce8d7dd5e77937793ba3
Remove code duplication

Remove code duplication from impl_mongodb and impl_db2 as both of
these drivers use pymongo. This is a multiple step process as there
are several differences between the implementation of these drivers.

The first step is about to move the identical functions to a common
base file, called pymongo_base with a Connection class, from which
the Connection class of both drivers will be inherited.

Change-Id: Ib3c250213cda5450c96c6e495f56623ed7ac0490
Merge "Implements monitoring-network"
Merge "rename meter table to sample"
Merge "sample table contains redundant/duplicate data"